Red-emitting material and organic electroluminescent device made by using the same

ABSTRACT

A red emitting material consisting of porphyrin, wherein the fluorescence of porphyrin is utilized. Metalloporphyrin can also be used as the porphyrin. An organic electroluminescent element having a porphyrin containing light emitting. The porphyrin content in the light emitting layer may be 1.0% by weight or more. The light emitting layer functions as a hole injection and transport layer or an electron injection and transport layer. Alternatively, it may be possible to further provide a hole injection and transport layer or an electron injection and transport layer.

This application is a 371 application of PCT/JP97/02206 filed Jun. 26,1997.

1. Technical Field

The present invention relates to a red light emitting material and anorganic electroluminescence (hereinafter called “organic EL”) element.More particularly, the present invention r elates to a red lightemitting material using fluorescent light of porphyrin and an organic ELmade by using the material.

2. Background Art

An organic EL element has c characteristics of self-luminescence andthin thickness, and the element can be driven with a low voltage. Theprinciple of the organic EL element had already been known in the 60's;at that time, however, the luminescent efficiency was still low so thatthe emission could only be seen in a dark place.

However, since C. W. Tang et al. published an improved element, whichcould be seen even in a bright place with a high luminescent efficiency,in the Applied Physics Letter, 51, 913 (1987), the research anddevelopment of organic EL has rapidly proceeded.

At the beginning of the development of organic EL, a blue light emittingelement was placed emphasis on also several sorts of chemical compoundtherefor were suggested, because a blue emitting element having a highbrightness had not existed at that time. While, many green emittingmaterials existed, so there was no problem concerning the green emittingmaterial.

On the other hand, nevertheless a red light emitting material is anelemental requirement to realize a full color display; the number ofsorts thereof, which have been developed until now, is smaller incomparison to the other colors, so that it is now required rapidly todevelop the red light emitting material.

Conventionally, elements are known, for instance, which are disclosed inthe Journal of the Applied Physics, 65, 3610 (1989), in Japanese PatentPreliminarily Publication No. Hei6-9953, in the Preliminarily PreparedPaper for the 41th Spring Lecture Meeting of Applied Physics Society,p28-N-1. However, these elements have a low color purity in order to usethem as the red component of a multiple color display or a full colordisplay, because the light emission thereof contains a low wave lengthcomponent. Therefore, these elements are not sufficient for practicaluse.

By the way, the fact that the porphyrin derivative is useful for anelectronic material, particularly, for a hole transporting material canbe predicted from the property of phthalocyanine, which is called as“azaporphyrin” and has a similar construction to that of porphyrin. Theelectric characteristic of phthalocyanine has been well studied; manytrials for an organic functional material have been taken forphthalocyanine.

Further, it is also well known that phthalocyanine is useful as a holetransporting layer of the organic EL element, from which it can beguessed that porphyrin could be used as a hole transporting layer of theorganic EL element.

However, the conventionally known usage of porphyrin as an organicelectronic material is limited to the hole transporting function. Forinstance, an application of porphyrin for a hole transporting materialis mentioned in Japanese Patent Publication No. Hei01-7635, JapanesePatent Preliminarily Publication Nos. Sho63-295695, Hei-04-233194,Hei04-233195, Hei05-17765 and Hei06-215874, an application for anelectrochromic material is disclosed in Japanese Patent Publication Nos.Hei01-33808 and Hei01-33809; an application for an optical memory or anoptical conductive material is recited in Japanese Patent PreliminarilyPublication No. Hei07-278445. All of these techniques use the holetransporting function of porphyrin; there is no study however as to theuse of porphyrin as a light emitting material using its fluorescentcharacteristic.

In this manner, the usage of porphyrin for an organic electronicmaterial has been widely known, but the application of porphrin for alight emitting element using its fluorescent characteristic has neverbeen considered, much less it has not been known at all that porphyrinfunctions as a material for the organic EL element to realize a redlight emission with a high purity.

Disclosure of Invention

The present invention has been made to solve the task of the prior artmentioned above and has its purpose to provide a red light emittingmaterial which realizes a red light emission with a high purity and toprovide an organic EL element using the material.

The present inventors studied several sorts of material to realize a redemitting organic element having a high purity; then they paid attentionto a characteristic in that the porphyrin derivative has a strongabsorption called a “soret band” in the vicinity of 400 nm and shows afluorescence in the vicinity of 600 nm. As a request of studying to usethe fluorescent characteristic of porphyrin, which had not beenconsidered at all at that time, the inventors found that a red lightemitting EL element with a high purity can be produced by the porphyrinand completed the present invention.

That is to say, the red light emitting material according to the presentinvention has a characteristic in that the material is for use in anorganic electroluminescent element and consists of porphyrin.

Further, the organic electroluminescent element according to theinvention comprises a light emitting layer which contains porphyrin.

Furthermore, the other organic electroluminescent element according tothe invention comprises a hole transporting material, an electrontransporting material and a layer which contains porphyrin of 1.0% byweight or above.

The detail of the present invention will be explained below.

It should be noted that the “porphyrin” used in the present invention isa generic term for a chemical compound having its basic structure ofporphyrin where four pyrrole rings are combined by a methine group andit is obtained by substitution of hydrogen atoms existing around thebasic structure.

Sometimes such a definition is used that phthalocyanine is a sort ofporphyrin, however, phthalocyanine is excluded from the presentinvention. Since phthalocyanine, which is possessed in porphyrin, doesnot have the intended fluorescent characteristic, therefore it cannot bepreferably used.

The porphyrin according to the invention can be expressed, for instance,by the following generic formula (1):

(wherein, R1 to R12 in the formula represent, independently, hydrogen,halogen, substituted or non-substituted alkyl group, alkoxy group,aryloxy group, perfluoroalkyl group, perfluoroalkoxy group, amino group,alkylcarbonyl group, arylcarbonyl group, alkoxycarbonyl group,aryloxycarbonyl group, azo group, alkylcarbonyloxy group,arylcarbonyloxy group, alkoxycarbonyloxy group, aryloxycarbonyloxygroup, sulfinyl group, sulfonyl group, silyl group, carbamoyl group,aryl group, heterocyclic group, alkenyl group, alkynyl group, nitrogroup, formyl group, nitroso group, formyloxy group, isocyano group,cyanate group, isocyanate group, thiocyanate group, isothiocyanategroup, cyano group, or condensate of substituted or non-substitutedcycles in case of adjacent substituent groups thereof.

Further, the porphyrin for use in a light emitting material of organicEL element, particularly, a red light emitting material of organic ELelement in the present invention, is not limited to the porphyrinexpressed by the above-mentioned generic, formula (1) but may beprovided with natural porphyrin, a high polymer molecule derived fromthe above-mentioned porphyrin, or a metalloporphyrin that is a metalcomplex of these porphyrins.

More concretely, ethioporphyrin-I, deuteroporphyrin-IX,meso-porphyrin-IX, hematoporphyrin-IX, protoporphyrin-IX,coproporphyrin-I, coproporphyrin-III, uroporphyrin-I, uroporphyrin-III,chlorofluoroporphyrin, hematoporphyrin, deuteroporphyrin-IX2,4-di-acrylic acid, 2,4-diformyldeuteroporphyrin-IX,2,4-diacetyldeuteroporphyrin-IX, deuteroporphyrin-IX 2,4-disulfonicacid, phylloporphyrin-XV, pyroporphyrin-XV, rhodoporphyrin-XV,phylloerythrin, phaeoporphyrin-a5, tetoraphenylporphyrin, protoheme,etc. can be cited.

Furthermore, as the metal to be introduced into metalloporphyrin, Mg,Al, S, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Sr, Y, Zr,Nb, Mo, Rh, Pd, Ag, Cd, In, Sn, Sb, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Au,Hg, Tl, Pb, Bi, Pr, Yb, Th, etc. should be mentioned.

The light emitting layer consisting of porphyrin or the light emittinglayer obtained by mixing porphyrin with an organic carrier transportingmaterial can be formed by using a known method, such as a deposition, aspin coating, and a casting; thereby an element which realizes a redlight emission by applying a voltage can be obtained with a high colorpurity.

The gist of the construction of the organic EL element will be explainedbelow.

An organic EL element produces exciton by the recombination of holesinjected through a positive electrode and electrons injected through anegative electrode; the light generated when the exciton disappears isused. Therefore, in principle, the element has to have a hole injectionand transport material, and an electron injection and transportmaterial, and a light emitting material. However, some materials combinethese functions, so that the above-mentioned materials can be omitted asthe occasion may demand.

There are several ways for the construction of organic EL element.Basically, the construction may be provided by a pair of electrodes(anode and cathode) and a light emitting layer held by the electrodes.It may be possible to add a hole injection and transport layer and/or anelectron injection transport layer as occasion demands. The holeinjection and transport layer or the electron injection and transportlayer sometimes functions as a light emitting layer.

Furthermore, in order to improve the luminescent efficiency, it mayprovide a hole interrupting layer or an electron interrupting layer.

According to the organic EL element of the present invention, porphyrinis contained in the light emitting layer; however, in case that the ELelement comprises the hole injection and transport layer and/or theelectron injection and transport layer, porphyrin may be contained inthese layers. In this case, the porphyrin contained into these layersfunctions as an assistant for the red light emission.

Moreover, according to the invention, it is also possible to provide amixture layer (which works as a light emitting layer) where a holetransporting material, an electron transporting material and porphyrinare mixed together.

In this case, it is preferred that the content of porphyrin in theselayers is 1% by weight. Further, in case that the organic EL elementaccording to the invention is used independently, it is preferred tomake the content of porphyrin 3% by weight or above, in order to make avisual sense of red color. Furthermore, in order to use the element as afull color display element or a multiple color display element where acolor tone is attached importance, the content of porphyrin may bepreferably 5% by weight or above, because a higher red color purity isrequired in such a case.

According to the organic EL element of the present invention, it ispossible to adjust the red color purity without affecting theluminescent efficiency by changing the content of porphyrin . Forinstance, when the content of porphyrin is 40% by weight or above, theluminescent efficiency does not decrease so much, and a display elementhaving an extremely high color purity can be obtained.

The conventional color producing assistant reagent is used in such amanner that a small amount of the assistant reagent is doped into alight emitting layer. Contrary to this, according to the EL element ofthe present invention, a large amount of porphyrin can be doped in thelight emitting layer, etc. greatly exceeding over the conventional rangeof the dopant. Particularly, in such a case, it is presumed that theporphyrin serves other functions than that of the assistant reagent.

There is an example of a method for the EL element according to theinvention that a thin film is formed between electrodes directly or viaa carrier transporting layer, which can be obtained by deposition ofporphyrin compound, or spin coating or dipping with a material which isobtained by dissolving the porphyrin compound in a solvent medium or bydissolving the porphyrin compound in a solvent medium using a highpolymer molecule as an integrating agent.

The other method for forming the EL element can also be cited where theporphyrin compound is mixed into a known carrier transporting material.

It should be noted that porphyrin is an amphoteric compound; thus thelayer obtained in the above-mentioned way can hold all layers of thehole injection and transport layer, the electron injection and transportlayer and the light emitting layer.

In such a construction of the element, it is preferred that at least oneof the electrodes is supported by a base plate. There is no speciallimitation for the base plate, so that conventional materials for thebase plate of the organic EL element, such as glass, plastic, quartz andceramics, can be used.

As an anode of the organic EL element, an electrode having an electrodesubstance made of a metal, an alloy, an electric conductive compound anda mixture thereof, each having a large work function (4 eV or more), canbe preferably used. For instance, a metal such as Au, electricconductive transparent material, such as CuI, SnO₂, ZnO and ITO (Indiumoxide) can be cited as such an electrode substance.

The anode can be obtained by forming a thin film by treating theelectrode substance by deposition, spattering, or other conventionalmethods.

On the other hand, as a cathode of the organic EL element, and electrodeprovided with an electrode substance made of a metal, an alloy, anelectric conductive compound and a mixture thereof, each having a smallwork function (4.3 eV or less), can be preferably used. For instance,Na, Na-K alloy, Al, Mg, Li, Li alloy, Mg alloy, Al alloy, Mg/Ag alloy,In, etc. can be mentioned for the electrode substance of cathode.

The cathode can be obtained by forming a thin film treated with theseelectrode substance by deposition or spattering, etc.

Next, the hole transporting material for use in forming the holeinjection and transport layer is a chemical compound by which holes canappropriately be transported to the light emitting layer when holes areinjected from the anode. The hole transporting material is not speciallylimited so far as the material has the above desired characteristic; sothat the material used as a carrier transporting material for hole outof photoconductive materials, or a material selected from knownmaterials used for the hole transporting layer of an organic EL elementcan be used.

The electron transporting material for use in forming the electroninjection and transport layer also is not specially limited so far asthe material has a function that transports the electrons injected froma cathode into the light emitting material; so that an arbitrarymaterial selected from known chemical compounds which have beenconventionally used as organic EL material can be used.

Then, as an example of preferred methods for manufacturing an organic ELelement according to the present invention will be explained.

First, the method for manufacturing an EL element comprisinganode/porphyrin compound/cathode will be explained. An anode is formedin such a manner that a thin film having its thickness of 1 μm or less,preferably 10 to 200 nm, made of a desired electrode material, forinstance a substance for anode is formed on an appropriate base platewith the aid of deposition or spattering, etc.; then a thin film made ofporphyrin is formed on the anode to provide a light emitting layer.

In this case, the light emitting layer may be formed only of porphyrinor of mixture of a luminescent mother material of carrier transportingmaterial, such as hole transporting material or electron transportingmaterial, and porphyrin. It should be noted that in order to obtain apreferred red color purity and a high luminescent efficiency, it isdesired to mix the porphyrin with other material.

As methods for forming a thin film of a light emitting material for sucha layer, for instance, a dipping method, a spin coating method, adeposition, etc can be used: any method can be used to form theporphyrin thin layer.

After forming the luminescent layer, i.e. a porphyrin layer or a layercontaining porphyrin, another thin layer made of a substance for cathodeis formed thereon by deposition or spattering to have a cathode layer,so that a desired organic EL element can be obtained.

It should be noted that the above-mentioned order to manufacture theorganic EL element can be reversed; that is to say, it is possible tomanufacture the element in the order from the cathode, the lightemitting layer, and the anode.

Then, the method for manufacturing an organic EL element comprising ananode/hole injection transporting·luminescent layer made of porphyrin orcontaining porphyrin/electron injection and transport layer/cathode willbe explained.

The anode and the porphyrin layer, etc. are formed in the same manner asthe above-mentioned method for manufacturing the organic EL element; theelectrron transporting layer is formed by deposition etc., as well asthe above case; then cathode is formed.

The layer containing porphyrin can be formed in such a manner that anappropriate amount of porphyrin and a material having a holetransporting function or high polymer molecular integrating agent aredissolved in a solvent, and then spin coating or dipping the thusobtained material. Or, the thin film may be provided by flash depositionor co-deposition from a plurality of deposition sources.

As well as the above case, it is possible to manufacture the element ina reversed order.

In addition, the elements having other constructions, such as anode/holetransporting layer/light emitting layer containing porphyrin/electrontransporting layer/cathode, and anode/hole transporting layer/electroninjection transporting·light emitting layer containingporphyrin/cathode, can be manufactured in the same manner as the above.

It should be noted that porphyrin can also be used for a red color lightemitting material of the organic EL element obtained by a color exchangemethod, which is mentioned in the Japanese Patent PreliminarilyPublication Nos. Hei5-2558860 and Hei6-203963.

When a direct or an alternating voltage (about 2 to 40V) is applied onthe thus obtained EL element, a light emission can be observed from thetransparent or semi-transparent electrode side.

For practical use, a luminescent intensity more than 10 cd/m² isnecessary: the EL element according to the present invention can beachieved at this level.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a graph showing an EL spectrum in an example of the organic ELelement according to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be explained in more detail on the basis ofembodiments; however, the invention is not limited to the embodiments.

Embodiment 1

A glass plate having a desired patterning shape with an ITO electrode isfixed on a base plate holder of a deposition apparatus, which isavailable from a market; N, N′-diphenylene-N, N′-bis-(3-methylphenyl)-4,4′-diamine(TPD) is deposited on the fixed base plate from a cruciblemade of quartz at a degree of vacuum of 8×10⁻⁴ Pa and a deposition speedof 0.1 to 0.2 nm/sec to obtain a hole injection and transport layerhaving a thickness of the film of 50 nm formed across the ITO electrodeand the glass base plate.

Then, 5,10,15,20-Tetraphenyl-21H,23H-porphine (TPP;tetraphenylporphyrin) is deposited from a crucible made of quartz at adegree of vacuum of 8×10⁻⁴ Pa and a deposition speed of 0.1 to 0.2nm/sec to obtain a layer having a thickness of the film of 50 nm, whichworks as an electron injection and transport layer and a light emittinglayer. Then, magnesium and silver are co-deposited on the layer, whichworks as an electron injection and transport layer and a light emittinglayer, via a shadow mask at a degree of vacuum of 1×10⁻³ Pa, and at adeposition speed of 1 to 1.2 nm/sec. and using a crucible made ofgraphite for magnesium, while at a deposition speed of 0.08 to 0.11nm/sec and using a crucible made of graphite for silver to provide amagnesium/silver electrode having a film thickness of 150 nm.

When a direct voltage was applied to the EL element with the ITOelectrode as an anode and the magnesium/silver electrode as a cathode, ared light emission with chromaticity coordinates of X=0.73, Y=0.27 wasobtained. The brightness thereof was 60 cd m² when the applied voltagewas 10V.

Embodiment 2

A hole injection and transport layer is formed on a glass base platewith ITO electrode by repeating the operation mentioned above; TPP andTris (8-hydroxyquinolinato) Aluminum (Alq₃) are co-deposited at a degreeof vacuum of 8×10⁻⁴ Pa; at a deposition speed of 0.02 to 0.04 nm/sec andusing a crucible made of quartz for TPP, while at a deposition speed 0.1to 0.2 nm/sec and using a crucible made of quartz for the Tris(8-hydroxyquinolinato) Aluminum (Alq₃) to obtain a light emitting layerhaving a film thickness of 50 nm.

Then, a magnesium/silver electrode is formed on the light emitting layerin the same manner of the embodiment 1. When a direct voltage is appliedto the thus obtained EL element as well as the embodiment 1, a red colorlight emission with a chromaticity coordinates of X=0.73, Y=0.27 wasobtained. The brightness thereof was 40 cd m² when the applied voltagewas 6V.

Embodiment 3

An organic EL element was manufactured by repeating the same operationof the embodiment 2 but a zinc substituent TPP was used instead of TPP.When a direct voltage was applied to this EL element in the same manneras the embodiment 1, a reddish orange color light emission withchromaticity coordinates of X=0.67, Y=0.33 was obtained. The brightnessthereof was 50 cd m² when the applied voltage was 10V.

Embodiment 4

A hole injection and transport layer and a light emitting layer weresubsequently formed on a glass base plate with ITO electrode byrepeating the same operation as that of the embodiment 2. Then, 1,1-dimethyl-2, 5-di(2-pyridil)-3, 4-diphenylsilacyclopentadiene isdeposited from a crucible made of quarts at a degree of vacuum of 8×10⁻⁴Pa and at a deposition speed of 0.1 to 0.2 nm/sec. to form an electroninjection and transport layer having a film thickness of 50 nm on thelight emitting layer.

Then, a magnesium/silver electrode was formed on the electron injectionand transport layer in the same manner as in the embodiment 1. When adirect current was applied to the thus obtained EL element in the samemanner as the embodiment 1, a red color light emission with achromaticity coordinates of X=0.73, Y=0.27 was obtained. The brightnessthereof was 50 cd m² when the applied voltage was 9V.

Embodiment 5

A thin film having a thickness of 100 nm made of a material obtained byspin coating a liquid obtained by dissolving 1 weight part ofpolyvinylcarbazole, 1 weight part of 2,5-bis(1-naphthyl)-1,3,4-oxadiazol and 0.4 part per weight of TPP in1,2-dichloroethane was formed as an light emitting layer on a glass baseplate with an ITO electrode, which is shown in the embodiment 1, and thethus obtained film was fixed to a base plate holder of the depositionapparatus.

Then a magnesium/silver electrode was formed on the light emitting layerin the same manner as in the embodiment 1. When a direct voltage wasapplied to the thus obtained EL element, a red light emission could beobtained.

Embodiment 6

A light emitting layer was formed by repeating the same operation as inthe embodiment 5, then the supporting plate was fixed on a base plateholder of the deposition apparatus. Then, a layer made of Alq₃ wasformed on the light emitting layer as an electron injection layer bydeposition from a crucible made of quartz at a degree of vacuum 8×10⁻⁴Pa and a deposition speed of 0.1 to 0.2 nm/sec. to obtain an electroninjection and transport layer having a thickness of 20 nm.

Thereafter, a magnesium/silver electrode was formed in the same manneras the embodiment 1. When a direct voltage was applied to the thusobtained EL element, a red light emission could be obtained.

Embodiment 7

After forming a hole transporting layer by repeating the same operationof the embodiment 1, tetraphenylporphyrin and Tris(8-hydroxyquinolinato) Aluminum were co-deposited at a degree of vacuumof 8×10⁻⁴ Pa; at a deposition speed of 0.03 to 0.04 nm/sec and using acrucible made of quartz for the tetraphenylporphyrin, while at adeposition speed of 1.0 to 1.3 nm/sec. and using a crucible made ofquartz for Tris (8-hydroxyquinolinato) Aluminum to obtain a lightemitting layer having a thickness of 50 nm. Then, a magnesium/silverelectrode is formed on the luminescent layer in the same manner of theembodiment 1. When a direct voltage is applied to the thus obtained ELelement as well as the embodiment 1, a red color light emission with achromaticity coordinates of X=0.67, Y=0.31 was obtained. The brightnessthereof was 40 cd m² when the applied voltage was 20V.

Embodiment 8

After forming a hole transporting layer by repeating the same operationof the embodiment 1, tetraphenylporphyrin and Tris(8-hydroxyquinolinato) Aluminum were co-deposited at a degree of vacuumof 8×10⁻⁴ Pa; at a deposition speed of 0.2 to 0.3 nm/sec and using acrucible made of quartz for the tetraphenylporphyrin, while at adeposition speed of 0.2 to 0.3 nm/sec. and using a crucible made ofquartz for Tris (8-hydroxyquinolinato) Aluminum to obtain a lightemitting layer having a thickness of 50 nm. Then, a magnesium/silverelectrode is formed on the light emitting layer in the same manner ofthe embodiment 1. When a direct voltage is applied to the thus obtainedEL element as well as the embodiment 1, a red color light emission witha chromaticity coordinates of X=0.73, Y=0.27 was obtained. Thebrightness thereof was 200 cd m² when the applied voltage was 12V.

EL spectrums for each EL element obtained in these embodiments is shownin FIG. 1.

Industrial Applicability

As explained above, according to the invention, a red light emittingmaterial realizing a red color light emission with a high color purityand an organic EL element using the material can be provided by using afluorescent characteristic of porphyrin.

That is to say, the organic el element according to the invention canemit a red color light with a high color purity; therefore, a displayelement having a high color reproducibility can be obtained by using theorganic el element according to the invention as a red color emittingsource for a full color plane light emitting element or a luminescentelement for displays.

What is claimed is:
 1. An organic electroluminescent element comprisinga red color light emitting layer which contains porphyrin (excludingphthalocyanine and octaethylporphyrin).
 2. An organic electroluminescentelement according to claim 1, wherein said light emitting layerfunctions as a hole injection transporting layer or an electroninjection and transport layer.
 3. An organic electroluminescent elementaccording to claim 1, wherein said light emitting layer containsporphyrin of 1.0% by weight or above.
 4. An organic electroluminescentelement according to claim 2, wherein said light emitting layer containsporphyrin of 1.0% by weight or above.
 5. An organic electroluminescentelement according to any one of claims 1 to 3, or 4, wherein a holeinjection and transport layer and/or an electron injection and transportlayer is further provided.
 6. An organic electroluminescent elementaccording to claim 5, wherein said hole injection and transport layerand/or electron injection and transport layer contains porphyrin.
 7. Anorganic electroluminescent element according to claim 6, wherein saidhole injection and transport layer and/or electron injection andtransport layer contains porphyrin of 1.0% by weight or above.